How can you check if a fragment of a non absorbable stitch was left in the skin? Ultrasound? The stitches were removed 1.5 months ago

Ultrasound is. unlikely to reveal anything. Most of the time this foreign body will work itself to the surface and fall out on its own. If it is particularly bothersome you could ask your health care professional to tweeze or needle it out like you would for a splinter.